A number of electrical transformers are known which transfer electrical energy by induction from one or more circuits to one or more circuits, at the same frequency and accomplishing usually a transformation in the voltage and current values.
Single-phase transformers are built with one coil and two cores or two coils and one core, and three-phase transformers are built with three coils and three or four cores. The purpose is to close electrical and magnetic circuits within the core and the coil, respectively; this type of transformer being the one most commonly used in industry.
The transformer core is built from magnetic steel, usually grain-oriented silicon steel, of different thicknesses, coatings and qualities. The greater the quality of core materials, the lesser are electrical losses.
Core losses are known as xe2x80x9cemptyxe2x80x9d or xe2x80x9cno-loadxe2x80x9d losses, since they are always present while the transformer is connected to electrical lines, regardless of whether there is or is not a load. These losses are measured in watts.
The coil manufacturing process consists in winding from several to thousands of turns of copper or aluminum wires or leads, either in the form of strip, of rectangular or round cross section, all of them with or without insulation. The common practice consists in winding first a low voltage conductor and thereafter one of high voltage, always observing the laws and traditional principles of the calculation of transformers and electrical machines. Traditional coil arrangements include schemes such as those of low voltagexe2x80x94high voltage and low voltagexe2x80x94high voltagexe2x80x94low voltage, depending on the needs of transformation required by the final user.
In transformer design, traditional electricity and magnetism formulas and criteria are used such as:
The transformation ratio, which results from the number of turns of the high voltage section (primary) to the number of turns of the low voltage section (secondary);
The wire""s current capacity, in amperes per square millimeter;
The electromagnetic induction caused by the turns of conductors on the core cross section, measured in teslas;
The impedance in percentage caused by resistance of the conductors, in ohms; and
The transformer excitation current, characteristic of the core material and construction pattern.
On the other hand, transformers are manufactured with a voltage regulating section to adjust the voltage of transmission lines to the voltage required by the final user, known as the tap changer section, which can change the output voltage in a xc2x15% or any other specified percentage.
Transformers are placed within a steel tank following the geometry of each construction, so that for single-phase transformers, the tank is generally round in cross section and for three-phase transformers the tank cross section is generally rectangular. Additionally, tanks are fitted with a series of accessories such as high voltage bushings, low voltage bushings, tap changer, ground plate or bolt, a combination of oil drain and lower filter valve, pressure bleeder and relief valve, overhead connection for leak test, transformer lifting hooks, name plate with the serial number and cooling radiators, as is well known in the art.
Depending on the user needs, there are self-protected transformers, both in high and low voltage, therefore accessories are added such as fuses, breakers, fault indicative lights, etc.
Another type of transformer is known as padmount type, which contains the same above described components. The basic difference between these transformers is the tank form and the additional guard and control accessories normally added. Generally, these transformers are used in termination of lines (radial feed) and in networks with line continuation (loop feed).
Transformers are required to comply with a series of laboratory tests and construction criteria, i.e., they should meet or fulfill various Mexican and International standards or norms. Within the main applicable standards in this field, there can be mentioned the Mexican standards NMX J 116, NMX J 285, NMX J 284, NMX J 169; of the United States of America ANSI C 57 12.00 (1993), NEMA MW 1000; Canadian CSA: CS M91, C 227.1, C227.2, C 227.3, C 227.4, C 301.1 and C2 M91; and International IEC Publication 76 (1993).
In order to make economic comparisons among transformers, the transformer sales price must be taken into account and the indices of Power Utilities or Electricity Companies on the electricity generation cost per Kilowatt (KW),
In this way, the Evaluated Price is obtained by adding the transformer sales price plus the no-load losses cost and the load losses cost, according to the following formula:
Evaluated Price=Transformer price+$No-load Losses+$Load Losses, wherein:
$No-load Losses=Index vxc3x97No-load Losses
$Load Losses=Index cxc3x97Load Losses
Typically indices used in the world are $USD 2.00 per watt of load, index c, and $USD 4.00 per watt of no-load loss, index v.
Procurement of transformers by power utilities or electrical companies is generally done by means of bidding, in which the manufacturer that has the smallest evaluated price is certainly the one who is designated as the supplier.
For many years, development in transformer design has been directed only to the improvement of construction materials in themselves. Regarding the cores, there exists better silicon steel strip; concerning electrical insulation materials, power factor and voltage resistance test Characteristics have been improved through additives and resins.
Currently, the most outstanding development in materials has been the invention of amorphous steel, which reduces up to 80% the normal no-load losses; however this great advantage is diminished for the following reasons:
Thickness hardly reaches 0.10 mm (0.004xe2x80x3), so the core manufacturing constitutes one of the most important problems to solve;
The stacking factor is of 82% maximum, while in silicon steel it is about 97%;
The magnetic saturation upper limit is of 13.5 teslas against 17 teslas of silicon steel;
It has less density than silicon steel, i.e., 7.18 g/cm3 against 7.65 g/cm3; and
The difference in price for steels is of $USD 1.80 per kilogram of silicon steel against $USD 4.08 per kilogram of amorphous steel.
Referring now to cores, the material used in their construction is typically grain-oriented silicon steel sheet, in different thicknesses, basically consisting of a low carbon iron-silicon alloy and coated on both faces with an insulating material known as xe2x80x9cCarlitexe2x80x9d or with glass fiber. Currently there are three principal types of cores: the Wescore core, the cruciform core and the toroidal type core.
The Wescore core was originally developed by Westinghouse in the sixties. This type of core permits large production volumes since there are generic machines available in the market. This type of core is found generally either in pole type and substation type single-phase or three-phase transformers. This type of core is also known as xe2x80x9cDistributed Air-Gap Wound Corexe2x80x9d.
The Wescore core is formed from a wound steel strip in a continuous form to which sequential cuts are realized in order to allow it to be disassembled and reassembled around the coil. In other words, coils and cores are manufactured in two separate processes, the core being reassembled thereafter on the coil thanks to the cuts realized on the core sheets. The cross section of this type of core is generally rectangular. The foregoing allows a high volume of production. The maximum voltage recommended for transformers manufactured with this type of core is about 69,000 volts and up to 3000 KVA, if it is a shell-type transformer.
On the other hand, the toroidal-type core is formed from a steel strip wound in continuous circular form, without cuts. The conductors are wound around the core, also forming a toroid. This pattern allows the core magnetic path and the winding electrical path to be kept closer, and in the core no losses exist due to cuts, for example, as those found in the Wescore core. The result of using a toroidal-type core is an efficient transformer with a dramatic reduction in total losses. To a greater extent, advantages of toroidal-type core transformers are, among others, low core losses, lower noise level, lower telephonic interference, greater support in short circuit and excellent thermal characteristics.
Finally, there is the cruciform core, which is generally formed from several plates cut and stacked, of one measure per leg and one or two measures per yoke, with cross-shaped cross section. This type of core is used in distribution and power transformers. The construction of this type of core presents a great advantage for power transformers but at low throughput. Their use is recommended only above 2,500 KVA.
Therefore based on the above, it is an object of the present invention to provide a core and coil assembly that complies with all Domestic and International requirements and standards and at the same time presents significant material savings.
It is another object of the present invention to provide a core and coil assembly, which, when incorporated in a transformer, significantly improves the evaluated price thereof by considerably reducing the no-load losses and load losses.
In a preferred embodiment of the invention a Wescore type core for a transformer is characterized in that the wounded steel strip forming said core body present differences in height so that a straight or progressive slope is formed defining one or more inclined or curved walls. The slope is such that the cross section area of the joining of two adjacent cores is an octagon, hexagon or even an oval or circle.
In a second embodiment of the invention a toroidal-type core for a transformer is disclosed, characterized in that the wound steel strip forming the core body presents a gradual decrease in its width so that a straight and progressive slope is formed defining one or more inclined or curved walls. The width decrease is such that the cross section area of the toroidal-type core is an octagon, hexagon or even an oval or circle.